One of the first steps in any settlement improvement project is the preparation of a base map. Whenever there are no recent maps of the project area, the available (old) maps need to be updated. This can be done by field survey, but this tends to be rather time consuming and expensive. An alternative is the use of small format aerial photography (SFAP), which provides an efficient means for producing an updated map of the project area.
This exercise focuses on creating a photo mosaic of an urban area in Dar-Es-Salaam, Tanzania. In the area covered by the mosaic height differences do exist. The direct linear transformation is used to correct the aerial photographs geometrically for relief displacement. This can only be done when a Digital Terrain Model (DTM) is available. In this exercise the DTM is derived from a contour map and the photos are georeferenced with GPS field observations and image-to-image registration. After this process the maps are resampled and glued together to form a mosaic of the area.
When observing the final result some irregularities are visible, e.g. a tire of the airplane. To avoid these irregularities two methods can be applied. One is by changing the order in the gluing process in such a way that an image with such an irregularity will be overlayed with another image. The other method is creating sub maps with the SubMap of Raster Map operation, cutting off the undesired parts.
Among the main applications of SFAP, updating plays an important role, particularly providing a record of the actual situation in areas that are changing continuously and fast, e.g. unplanned residential areas in cities in developing countries.
A mosaic made from a series of small format aerial photos can provide a means for the production of an updated map of such an area, but also of (for instance) a disaster area (flooding, earthquake, etc.).
A number of other potential applications are evident and regularly practiced.
To illustrate the point, the exercise SFAP mosaic of the Keko Mwanga area in Dar-Es-Salaam, Tanzania, actually has been used for the following activities:
- to update the 1992 topographic base map of the settlement, primarily for delineating new and modified buildings;
- to identify and prioritize the problems that local residents are faced with in this area. The mosaic has proved to be very useful in group sessions, as persons easily understand it with little or no previous exposure to maps or aerial photographs;
- in discussions by the community on the location of an extension to the primary school;
- to locate solid waste collection points by an NGO working in the area;
- improvement of the building registration system, which is the basis for local revenue collection, by the local government.
- Hofstee, P. (1984). Small format aerial photography: simple and cheap do-it-yourself technique. In: Cities, vol. 1 no.3, Feb 1984, pp. 243-247.
- Janssen, L.L.F. (ed.) (2000). Principles of Remote Sensing: an introductory textbook. ITC, Enschede, 170 pp. ISBN 90-6164-183-7.
- Küffer, M., R. Sliuzas and G. Huurneman (2001). Estimating the consolidation levels of informal settlements in Dar Es Salaam with SPOT and ERS images. Paper 2nd Symposium Remote Sensing of Urban Areas, 22-23 June 2001, Regensburg, Germany.
- Warner, W. S., R. W. Graham, et al. (1996). Small Format Aerial Photography. Caithess: Whittles, 1996. 348 pp. ISBN 1-87032.
For information on this case study, contact:
A. Nagelhout and P. Hofstee
Department of Urban and Regional Planning and Geo-information Management,
International Institute for Geo-Information Science and Earth Observation (ITC),
P.O. Box 6, 7500 AA Enschede, The Netherlands.
Tel: +31 53 4874237, Fax: +31 53 4874399, e-mail: email@example.com